Methods for Detecting Petroleum Substances in Water — Detailed Explanation of the Fluorescence Method

Pollution of water bodies by petroleum substances has become one of the major global environmental concerns. From industrial wastewater discharge to marine tanker oil spills, these pollutants not only disrupt ecological balance but also threaten human health and fisheries resources. With increasingly stringent environmental protection regulations, such as China’s Environmental Quality Standards for Surface Water (GB 3838-2002) and marine environmental protection standards, oil-in-water monitoring has become a routine monitoring item. Common detection methods include gravimetric methods, infrared methods, ultraviolet methods, chromatographic methods, and fluorescence methods. Among them, the fluorescence method stands out for its efficiency and real-time capabilities. This article begins with the hazards of petroleum substances, systematically introduces various detection methods, and focuses on the principles and applications of the fluorescence method. As a reliable partner in the field of water quality sensing, Nexisense fluorescence products have demonstrated stable performance in practical monitoring, helping users respond promptly to pollution risks.

Main Hazards of Petroleum Substances in Water

After entering water bodies, petroleum substances can exist in various forms: floating oil films, dispersed emulsions, or dissolved states. These forms not only alter the physical properties of water but also trigger a chain of ecological effects. The primary hazard is the consumption of dissolved oxygen (DO), which further deteriorates water quality. Secondly, petroleum substances exert toxic effects on aquatic organisms, especially fish, directly damaging fisheries production chains.

Consumption of DO, leading to water quality deterioration
Dispersed oil droplets easily adsorb onto suspended particles or remain suspended in water in emulsified form. Microorganisms require large amounts of oxygen to decompose these organic substances, causing a sharp decline in DO. Meanwhile, surface oil films block gas exchange between air and water, hindering oxygen replenishment. This dual mechanism accelerates hypoxia, induces black and odorous water conditions, and may even create dead zones. In rivers or lakes, long-term accumulation can intensify eutrophication, triggering algal blooms that further consume oxygen and form a vicious cycle.

Toxicity to fish, damaging fisheries production
Petroleum substances are particularly toxic to fish. Even at concentrations as low as 0.1–0.01 mg/L, they can interfere with fish egg hatching, reduce survival rates, and increase deformity rates. Oil films and oil agglomerates can adhere to fish eggs and larvae, blocking gill respiration and causing suffocation. Historical cases, such as petroleum pollution in the Caspian Sea of the former Soviet Union, led to sharp declines in plankton populations, a two-thirds reduction in sturgeon yields, and near extinction of carp and roach. Globally, similar incidents occur frequently, impacting fisheries economies, reducing biodiversity, and threatening food chain stability.

Overview of Common Oil-in-Water Monitoring Methods

To effectively monitor petroleum pollution, environmental monitoring has developed multiple methods based on different principles, suitable for laboratory analysis or field detection. The following introduces six mainstream techniques, each with its own focus.

Gravimetric Method: Basic Extraction and Weighing

The gravimetric method extracts mineral oil from water samples using organic solvents (such as carbon tetrachloride), then evaporates the solvent and weighs the residue. This method is simple and intuitive and suitable for high-concentration samples, but it is time-consuming, susceptible to interference, and unable to distinguish oil types. It is often used as a reference method in laboratories but is not suitable for real-time monitoring.

Infrared Method: Vibrational Absorption Quantification

The infrared method utilizes the absorption characteristics of CH₂ and CH₃ groups in oils at specific wavelengths (2930 cm⁻¹, 2960 cm⁻¹) to calculate content based on absorbance. Since 1997, this method has been established as a Chinese national standard (GB/T 16488-1996) for mineral oil determination. Its advantages include high sensitivity and good reproducibility, but it requires specialized instruments and is easily affected by water turbidity.

Ultraviolet Method: Absorption Spectral Analysis

The ultraviolet method targets aromatic hydrocarbon compounds with conjugated double bonds, measuring absorption in the 220–280 nm range and converting oil content using standard curves. This method is easy to operate and low in cost, but it has insufficient sensitivity to non-aromatic oils and is easily affected by organic interference, making it suitable mainly for preliminary screening.

Chromatographic Methods: Separation, Qualitative and Quantitative Analysis

Chromatographic methods are based on differences in component distribution between stationary and mobile phases. Gas chromatography (GC) or high-performance liquid chromatography (HPLC) generates chromatograms for qualitative identification and quantitative calculation. These methods offer high accuracy and component differentiation but require expensive equipment and complex sample pretreatment, making them suitable for in-depth laboratory analysis.

Fluorescence Method: Energy Emission Measurement

The fluorescence method exploits the property of petroleum substances (such as polycyclic aromatic hydrocarbons) to emit fluorescence after absorbing ultraviolet light. When excited by a light source, fluorescence intensity is positively correlated with oil concentration. This method features rapid response and strong selectivity, is especially suitable for low-concentration detection, and supports online monitoring.

Other Methods: Supplementary Techniques

Turbidity methods measure turbidity based on light scattering by oil droplets but offer low accuracy; TOC (total organic carbon) methods evaluate total carbon content but cannot specifically identify oil; pyrolysis and resistance methods are used in specific scenarios such as industrial process control. These methods often serve as supplements to mainstream techniques like fluorescence.

In-Depth Analysis of Fluorescence Method Principles and Advantages

The fluorescence method is based on quantum mechanical energy transition principles. After absorbing photons of specific wavelengths (usually in the ultraviolet range), electrons in petroleum organic molecules transition to excited states and release energy as fluorescence when returning to the ground state. Fluorescence intensity is proportional to molecular concentration. Fluorometers capture the signal and calculate content using calibration curves.

Compared with other methods, the fluorescence method is efficient and fast, with detection limits reaching the ppb level, good repeatability, and minimal interference from turbidity or color. It requires no complex pretreatment and has response times of only seconds, making it particularly suitable for continuous monitoring. However, fluorescence quenching effects (such as self-absorption at high concentrations) should be considered, and periodic calibration is required to maintain accuracy. In marine and river monitoring, this method can capture leakage events in real time and provide early warnings.

Applications of Nexisense Fluorescence Oil-in-Water Sensors

Nexisense fluorescence oil-in-water sensors adopt ultraviolet excitation principles and integrate self-cleaning brush designs to effectively eliminate interference from bubbles and fouling, extending maintenance intervals. These sensors support digital output and remote transmission and are suitable for monitoring industrial wastewater discharge outlets, river sections, and offshore platforms. In practical cases, such as wastewater treatment plants, they help optimize processes and prevent DO depletion caused by oil accumulation; in fisheries areas, they provide pollution early warnings to protect aquatic resources. Over long-term use, the sensors maintain high stability, helping users transition from passive response to proactive prevention.

Frequently Asked Questions

Which is more suitable for on-site monitoring, fluorescence or infrared methods?
The fluorescence method offers fast response and high portability, making it suitable for real-time online monitoring. Infrared methods provide high accuracy but require laboratory equipment and are more suitable for standard measurements.

Which method is most sensitive at low oil concentrations?
The fluorescence method has detection limits down to the microgram level, outperforming gravimetric and turbidity methods, especially in low-concentration pollution scenarios.

What is the toxicity threshold of petroleum substances to fish?
Concentrations of 0.1 mg/L can already affect fish egg survival, while at 0.01 mg/L deformity rates increase significantly, requiring strict monitoring.

How do self-cleaning sensors reduce interference?
Built-in brushes periodically remove fouling and bubbles, ensuring a clean optical path and improving data reliability.

Conclusion: Choosing the Fluorescence Method to Enhance Efficient Water Quality Monitoring

The hazards of petroleum substances in water cannot be ignored, from DO depletion to ecological damage, each stage demanding accurate monitoring technologies. Traditional methods such as gravimetric and infrared techniques have laid the foundation, while the fluorescence method, with its speed and stability, is leading monitoring toward intelligent transformation. Nexisense fluorescence sensors have proven their reliability and ease of use in practical applications, effectively supporting pollution prevention and control. In the future, as sensors integrate with the Internet of Things, water body protection will become more proactive. Choosing the right method ensures that monitoring data truly safeguard clean water environments.